Aluminum-plutonium alloys



June s, 1965 T. 1. JONES 3,188,202

ALUMINUM-PLUTONIUM ALLOYS Figure 1. A1 S wt.% Pu Alloy-- Control Lightphase: A1 dendrites Dark phase: Al/PuAlh eutectc Wilma un Figure 3.A1+Swt.%Pu+ 1S Win55 Si Alloy Figure l1. A1 S :913.55 Pu Figure 5. A1 2Owt.% Pu Alloy 1S wh! U Alloy X i420 X h2o Lghb phase: PuAl Dark phase:eutectic 3,183,202 ALUMINUM-PLUTONIUM ALLOYS Thomas Ivor .i ones, DeepRiver, Ontario, Canada, assigner to Atomic Energy of Canada Limited,Ontario, Canada, a corporation n Filed Apr. 19, 1961, Ser. No. 104,183 7Claims. (Cl. 75-122.5)

This invention relates to alloys of aluminum and plutonium, and tomethods for their preparation. In particular, the invention includesternary alloys of aluminum, plutonium and one element selected from thegroup consisting of uranium, silicon and nickel.

Considerable attention is being devoted to the development of nuclear.power reactors which W-ill produce e-lectrical power competitively Withthe other conventional energy sources. One type of power reactor whichis being investigated is water-cooled, in which pressurized water orsteam at elevated temperatures (e.g., 250-400 C) is circulated aroundthe fuel rods. The fuel rods are sheathed in a suitable strong,corrosion-resistant alloy (e.g., Zircaloy-Z). It is important that, inthe event of sheath failure, the coolant :system should not become ex- Ycessively contaminated by the corrosion of the nuclear fuel.

Binary Al-Pu alloys have been found suitable for use as fuel in researchreactors operating with low temperature water. The corrosion resistanceto high temperature water of Al alloys containing about 13-20 wt.percent Pu and of hyper-eutectic structure, has been found to be good.However, Al alloys containing less than 13 percent Pu and ofhypo-eutectic structure, undergo rapid corrosion attack by hightemperature water. (T. I. Jones, Second International ConferencePlutoniurn Metallurgy, Grenoble, France, April 1960, IV Session, Paper1.)

Rod-type fuel elements in water-cooled power reactors, would normally beenriched with less than 13 wt. percent Pu. Al-Pu alloys of up to 13 Wt.percent Pu would be of value as fuel elements if they were resistant tocorrosion by water at up to about 350 C.

An object of this invention is to prepare a corrosionresistantaluminum-plutonium alloy of less than 13 wt. percent Pu. Another objectis to provide such an alloy which does not absorb neutrons to asignificant degree.

I have now found that Al alloys with up to about 8 percent Pu will formternary alloys with one of the group consisting of nickel, silicon anduranium, which ternary alloys have good corrosion-resistance to hightemperature water.

'The proportion of Pu in the alloys, Agiving 1a suitable enriclrment,lmay range from about 2 `to about 8 wt. percent. Four to six wt. percentPu is presently preferred. The nickel has a relatively largeneutron-capture cross-section and its proportion is desirably kept low.From 1 to 2 wt. percent Ni is suitable. The proportions of silicon anduranium fall within the range 13-17 wt. percent, with the total Pu-l-Sior total Pu-i-U preferably being within the range 19-21 wt. percent. Thehyper-eutectic structure and the absence of any signicant free Al phaseis ensured at these proportions. Zirconium was found to be unsuitable asa ternary additive since the `corrosion resistance to high temperaturewater was decreased from that of the hypo-eutectic binary alloy.

The alloys are preferably prepared by reducing plutonium dioxide withexcess aluminum metal in the presence of cryolite (3NaF.AlF3) at aboutl100-1200 C., to produce billets of the desired composition. Thesebillets are remelted and the temperature raised to about (a) 775 C.before making the Ni addition, (b) 850 C. before making the Si addition,or (c) 950 C. before mak-ing the U addition, The Ni, Si, and U arepreferably added in their elemental form. The above preferred tempera-3,188,202 Patented June 8, 1965 tures of the ternary addition are notcritical and may be increased or decreased by about 10 C. to 20 C. ormore. However, if the temperatures are much outside these limitsnon-uniform dispersion, lack of dissolution, presence oct dissolvedgases, etc. may be the result.

The ternary alloys are then cast into moulds preheated to about C. (cg., graph-ite moulds). Other methods may be used for preparing theinitial binary alloy such as reducing plutonium trifluoride with excessaluminum or adding Pu metal to Al metal in the melt. (See Runnalls,Progress in Nuclear Energy, Series V, 2, 98 to 118, Pergamon Press,1959.)

The following examples will illustrate the invention. Binary alloy of Aland 5 wt. percent Pu was prepared by the reduction of Pu02 with excessAl, in the presence of cryolite, at 1l50 C. Ternary additions of 1 wt.percent Ni, 15 wt. percent Si and 15 wt. percent U were then made asdescribed above to separate portions of the binary alloy.

Corrosion tests were carried out on the alloys using static de-ionizedWater at 340 C. This would be a typical temperature of the fuel exteriorfor a power reactor element cooled by water at 270-280o C. and having a60- 70 C. temperature gradient across Zircaloy-Z sheathing. [Zircaloy-2Iis an alloy typically consisting of 1.5% Sn, 0.1% Fe, 0.1% Cr, 0.05% Cr,0.05% Ni and the ballance Zr.] Corrosion test specimens were machinedfrom cast rods, degreased in acetone, etched for one minute in 5% NaOHsolution and thoroughly washed in deionized water. The time of exposureto the high temperature water was 20 hours. 'Ihe corrosion `test resultsare summarized in the following Table.

Corrosion of Al-Pu alloys FIGURES 1 to 5 illustrate the 'as-caststructures of the alloys listed in the above table.

Tlhe Al-i-S wt. percent Pu alloy is typical of an alloy having ahypo-eutectic structure with considerable free Al phase -beingpresent-as shown in FIGURE l. This binary alloy is rapidly corroded. Thenickel alloy structure includes a hypo-eutectic structure with verysmall equiaxed grains off oc-Al distributed as shown in FIGURE 2. Itwill be observed in FIGURES 3-5 that the structures of the 20 wt.percent Pu alloy, the Si alloy and the U alloy, are all qui-te similarIbeing highly relined and hypereutectic. The corrosion resistance of thethree ternary alloys is similar to that of the 20 wt. percent Pu alloy.

The corrosion resistance of the ternary alloys is signicantly improvedrelative to the 5 wt. percent plutonium `binary alloy-such that theternary alloys are suitable from a corrosion standpoint :for fuelapplications using high temperature water coolant.

The above examples are not intended to be limiting. The proportions maybe varied as indicated afbove. Other methods of preparing the initialbinary Al-P-u alloy may be used. The uranium in the example was thenaturallyoccurring mixture of isotopes, but other isotope mixtures maybe used. In the above illustrative alloys, super-purity yaluminum Wasused. Spectrographic analyses of the ternary alloys revealed thefollowing impurities:

Weight percent Na 0.03 B, Cr, Fe, K', Pxb, Ca, Ti, Zn 0.02

However, it is not necessary `to use such rsu'peppurity aluminum.

I claim:

1. An alloy consisting of from 2 to 8 Wt. percent plu- Y tonium, uraniumfrom 13 to 17 Wt. percent, and Ythe balance aluminum.

2. The alloy of claim 1 wherein the plutonium is pres- Vent in from 4 to6 Wt. percent.

3. The alloy of claim 1 wherein the total'pl-utonium plus uraniumconcentration is from 19 to 21 wt. percent.

4. An alloy consisting of about 5 wt. percent plu-v tonium, alnout 1 Wt.percent nickel, and the bal-ance aluminum.

5. An alloy consisting of about 5 Wt. percent plutonium, about 15 wt.percent uranium, and the balance aluminum.

6. An `alloy consisting of from 2 to 8 Wt. percent plutonium, nickel infrom 1 to 2 Wt. percent, and the balance aluminum.

7. The `alloy of claim 6 wherein the plutonium is present in from 4 to 6Wt. percent.

References Cited by the Examiner UNTED STATES PATENTS 2,885,283 5/59Sohonlfeld' et al. 75-122.5 r2,919,186 12/59 Colbeck 75-l22.7 2,929,7063/60 Cramer et al. 75-122.7 y2,934,424 4/60 MacKenzie 75-84.1 2,992,9157/61 Nelson 75-84.1

CARL D. QUARFORTH, Primary Examiner. OSCAR R. VERTIZ, Examiner.

1. AN ALLOY CONSISTING OF FROM 2 TO 8 WT. PERCENT PLUTONIUM, URANIUMFROM 13 TO 17 WT. PERCENT, AND THE BALANCE ALUMINUM.
 6. AN ALLOYCONSISTING OF FROM 2 TO 8 WT.. PERCENT PLUTONIUM, NICKEL IN FROM 1 TO 2WT. PERCENT, AND THE BALANCE ALUMINUM.